Antenna assembly for supplying power to an implantable neural stimulator device

ABSTRACT

An antenna assembly includes a metal layer configured to emit linearly polarized electromagnetic energy to a receiving antenna implanted underneath a subject&#39;s skin; and a feed port configured to connect the antenna assembly to a signal generator such that the antenna assembly receives an input signal from the signal generator and then transmits the input signal to the receiving dipole antenna, wherein the antenna assembly is less than 200 um in thickness, and wherein the metal layer is operable as a dipole antenna with a reflection ratio of at least 6 dB, the reflection ratio corresponding to a ratio of a transmission power of the antenna assembly in transmitting the input signal and a reflection power seen by the antenna assembly resulting from electromagnetic emission of the input signal.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/019,094, filed Jun. 26, 2018, which is a continuation of U.S.application Ser. No. 14/986,324, filed Dec. 31, 2015, now U.S. Pat. No.10,058,705, issued Aug. 28, 2018, which claims the benefit of U.S.provisional Patent Application 62/098,946, titled ANTENNA ASSEMBLY, andfiled on Dec. 31, 2014. All of these prior applications are incorporatedby reference in their entirety.

TECHNICAL FIELD

This application relates generally to an antenna assembly to coupleenergy to an implanted stimulator device.

BACKGROUND

Modulation of excitable tissue in the body by electrical stimulation hasbecome an important type of therapy for patients with chronic disablingconditions, including pain, movement initiation and control, involuntarymovements, vascular insufficiency, heart arrhythmias and various othermodalities. A variety of therapeutic intra-body electrical stimulationtechniques can be utilized to provide therapeutic relief for theseconditions. For instance, devices may be used to deliver stimulatorysignals to excitable tissue, record vital signs, perform pacing ordefibrillation operations, record action potential activity fromtargeted tissue, control drug release from time-release capsules or drugpump units, or interface with the auditory system to assist withhearing.

SUMMARY

In one aspect, some implementations provide an antenna assembly thatincludes: a metal layer configured to emit linearly polarizedelectromagnetic energy to a receiving antenna implanted underneath asubject's skin; and a feed port configured to connect the antennaassembly to a signal generator such that the antenna assembly receivesan input signal from the signal generator and then transmits the inputsignal to the receiving dipole antenna, wherein the antenna assembly isless than 200 um in thickness, and wherein the metal layer is operableas a dipole antenna with a reflection ratio of at least 6 dB, thereflection ratio corresponding to a ratio of a transmission power of theantenna assembly in transmitting the input signal and a reflection powerseen by the antenna assembly resulting from electromagnetic emission ofthe input signal.

Implementations may include one or more of the following features.

The metal layer may be shaped as a rectangle, and wherein a long axis ofthe rectangle may align with a direction of the linear polarization ofthe electromagnetic energy. The metal layer may include four roundedfillets. The metal layer may be a two-leaf structure that includes twoleaves each having three vertices, wherein the two leaves may adjoineach other at one vertex, and wherein the remaining vertices of eachleaf may be rounded as fillets. The metal layer may be operable tocreate a higher electric field than a metal layer configured as otherthan the two-leaf structure while maintaining a surface area identicalto the two-leaf structure. The feed port may be located at the vertexwhere the two leaves adjoin each other.

The antenna assembly may be configured such that the antenna assemblycan be bent up to 50 degrees while maintaining the reflection ratio ofmore than 6 dB. The antenna assembly may be configured to emittranscranially the linearly polarized electromagnetic energy when theantenna assembly is worn as an ear piece. The antenna assembly mayconfigured to emit the linearly polarized electromagnetic energy to areceiving antenna implanted up to 6 cm underneath a subject's skin. Theantenna assembly may be configured such that the reflection ratio of atleast 6 dB is maintained regardless of a separation between the metallayer and a subject's skin. The antenna assembly may be configured suchthat the reflection ratio of at least 6 dB is maintained with an air gapand without gel coupling between the metal layer and the subject's skin.The antenna assembly may be configured to operate with a quality factor(Q) no more than 9. The antenna assembly may be configured to operate ata frequency between 800 MHz and 3 GHz.

In another aspect, some implementations provide a system that includes asignal generator configured to generate an input signal containingelectrical energy and stimulation pulse parameters, an antenna assemblycoupled to the signal generator and configured to receive an inputsignal from the signal generator and then transmit the same to areceiving dipole antenna of a passive stimulator device implantedunderneath the subject's skin such that the antenna assembly operateswith a reflection ratio of at least 6 dB, the reflection ratiocorresponding to a ratio of a transmission power of the antenna assemblyin transmitting the input signal and a reflection power seen by theantenna assembly resulting from electromagnetic emission of the inputsignal, the antenna assembly including: a feed port configured toconnect the antenna assembly to the signal generator such that theantenna assembly receives the input signal from the signal generator;and a metal layer less than 200 um in thickness and configured to emitlinearly polarized electromagnetic energy via radiative coupling to thereceiving dipole antenna such that the passive stimulator deviceextracts the electrical energy from the input signal and then uses theextracted energy to create stimulation pulses suitable for stimulatingtissue; and a passive neural stimulator device configured to beimplanted underneath the subject's skin, the passive neural stimulatordevice including: a receiving dipole antenna configured to receive theinput signal emitted from the antenna assembly; and circuitry coupled tothe receiving dipole antenna, the circuitry being configured to: extractelectric energy contained in the input signal; and use the extractedelectrical energy to create stimulation pulses suitable for stimulatingneural tissue, the stimulation pulses being created according to thestimulation pulse parameters.

Implementations may include one or more of the following features.

The antenna assembly may be configured to emit the linearly polarizedelectromagnetic energy to the receiving antenna implanted 1-3 cmunderneath a subject's skin. The metal layer may be shaped as arectangle, and wherein a long axis of the rectangle may align with adirection of the linear polarization. The metal layer may include atwo-leaf structure that includes two leaves each having three vertices,wherein the two leaves adjoins each other at one vertex, and wherein theremaining vertices of each leaf are rounded as fillets. The feed portmay be located on the adjoining vertex. The antenna assembly may beconfigured such that the antenna assembly can be to be bent up to 50degrees while maintaining the reflection ratio of more than 6 dB. Theantenna assembly may be configured such that the reflection ratio of atleast 6 dB is maintained with an air gap and without gel couplingbetween the metal layer and the subject's skin.

The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other features,objects, and advantages will be apparent from the description anddrawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a high-level diagram of an example of a wirelessstimulation system.

FIG. 2 depicts a detailed diagram of an example of the wirelessstimulation system.

FIG. 3 illustrates a transmitting antenna coupling electromagneticenergy to a receiving dipole antenna through human skin, according to asimulation model.

FIGS. 4A-4B show an example of a transmitting antenna configured as awide dipole antenna assembly for operation at 915 MHz.

FIG. 5 shows S-parameter results of the wide dipole antenna assembly ofFIGS. 4A-4B.

FIGS. 6A-6B show example radiation patterns of the wide dipole antennaassembly of FIGS. 4A-4B.

FIGS. 7A-7B show an example of a transmitting antenna configured as awide dipole antenna assembly for operation at 2.4 GHz.

FIG. 8 shows S-parameter results of the wide dipole antenna assembly ofFIGS. 7A-7B.

FIG. 9 shows example radiation patterns of the wide dipole antennaassembly of FIGS. 7A-7B.

FIGS. 10A-10C show an example of a transmitting antenna configured as abowtie antenna assembly.

FIGS. 11A-11B show an example of a bowtie antenna assembly and thecorresponding S-parameter.

FIG. 12A shows an example of a radiation pattern of the bowtie antennaassembly of FIGS. 10A-10C in a sagittal view.

FIG. 12B shows an example radiation pattern from FIG. 12A in a zoomedview and superposed with the profile of the receiving antenna.

FIG. 13 shows the example bowtie antenna assembly in position forradiative treatment in an axial view.

FIG. 14A shows an example of a radiation pattern of the bowtie antennaassembly of FIGS. 10A-10C in the axial view.

FIG. 14B shows the example of the radiation pattern of FIG. 14A in azoomed view and superposed with the profile of the receiving antenna.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

In various implementations, systems and methods are disclosed forapplying one or more electrical impulses to targeted excitable tissue,such as nerves, for treating chronic pain, inflammation, arthritis,sleep apnea, seizures, incontinence, pain associated with cancer,incontinence, problems of movement initiation and control, involuntarymovements, vascular insufficiency, heart arrhythmias, obesity, diabetes,craniofacial pain, such as migraines or cluster headaches, and otherdisorders. In certain embodiments, a device may be used to sendelectrical energy to targeted nerve tissue by using remote radiofrequency (RF) energy without cables or inductive coupling to power animplanted wireless stimulator device. The targeted nerves can include,but are not limited to, the spinal cord and surrounding areas, includingthe dorsal horn, dorsal root ganglion, the exiting nerve roots, nerveganglions, the dorsal column fibers and the peripheral nerve bundlesleaving the dorsal column and brain, such as the vagus, occipital,trigeminal, hypoglossal, sacral, coccygeal nerves and the like.

A wireless stimulation system can include an implantable stimulatordevice with one or more electrodes and one or more conductive antennas(for example, dipole or patch antennas), and internal circuitry forfrequency waveform and electrical energy rectification. The system mayfurther comprise an external controller and antenna for transmittingradio frequency or microwave energy from an external source to theimplantable stimulator device with neither cables nor inductive couplingto provide power.

In various implementations, the wireless implantable stimulator deviceis powered wirelessly (and therefore does not require a wiredconnection) and contains the circuitry necessary to receive the pulseinstructions from a source external to the body. For example, variousembodiments employ internal dipole (or other) antenna configuration(s)to receive RF power through electrical radiative coupling. This allowssuch devices to produce electrical currents capable of stimulating nervebundles without a physical connection to an implantable pulse generator(IPG) or use of an inductive coil.

In some implementations, a signal generator and an antenna assembly maybe configured as wearable by a subject. The signal generator maygenerate an input signal containing electrical energy and stimulationpulse parameters. The antenna assembly may be coupled to the signalgenerator, for example, through a feed port, to receive the inputsignal. The antenna assembly may be thin and flexible. For example, theantenna assembly may be less than 200 μm in thickness, and may be ableto be bent up to 50 degrees while remaining operational. In someimplementations, when the antenna assembly is worn by a subject, a metallayer of the antenna assembly faces the subject. This metal layer may beconfigured to emit linearly polarized electromagnetic energy to areceiving dipole antenna of a passive wireless stimulator device suchthat the wireless stimulator device extracts the electrical energy fromthe input signal and then uses the extracted energy to createstimulation pulses suitable for stimulating tissue. The metal layer maybe shaped as a rectangle, much like a wide dipole antenna describedherein. In this case, the long axis of the rectangular shape is alignedwith the polarization direction. The metal layer may also be a two-leafstructure that includes two leaves adjoining each other at a vertex. Inthis case, the two leaves may operate as a signal arm and a ground armat a given time. An axial direction of the leaves connecting the twoarms may correspond to the direction of the linear polarization. Ineither case, the antenna assembly may be configured to emit the linearlypolarized electromagnetic energy efficiently. For example, the antennaassembly may operate with a reflection ratio of at least 6 dB at anoperating frequency of the antenna assembly. The reflection ratiocorresponds to a ratio of a transmission power used by the antennaassembly to transmit the input signal and a reflection power seen by theantenna assembly resulting from electromagnetic emission of the inputsignal. The reflection ratio of at least 6 dB at an operating frequencyof the antenna assembly may be maintained regardless of a separationbetween the metal layer and the subject's skin. For example, the antennaassembly may be configured such that reflection ratio of at least 6 dBat the operating frequency of the antenna assembly is maintained whenthe antenna assembly is between zero to 2 centimeters, or moreparticularly, between zero to 1 centimeter, away from the patient'sskin.

Further descriptions of exemplary wireless systems for providing neuralstimulation to a patient can be found in commonly-assigned, co-pendingpublished PCT applications PCT/US2012/23029 filed Jan. 28, 2011,PCT/US2012/32200 filed Apr. 11, 2011, PCT/US2012/48903, filed Jan. 28,2011, PCT/US2012/50633, filed Aug. 12, 2011 and PCT/US2012/55746, filedSep. 15, 2011, the complete disclosures of which are incorporated byreference.

FIG. 1 depicts a high-level diagram of an example of a wirelessstimulation system. The wireless stimulation system may include fourmajor components, namely, a programmer module 102, a RF pulse generatormodule 106, a transmit (TX) antenna 110 (for example, a patch antenna,slot antenna, or a dipole antenna), and an implanted wireless stimulatordevice 114. The programmer module 102 may be a computer device, such asa smart phone, running a software application that supports a wirelessconnection 104, such as Bluetooth®. The application can enable the userto view the system status and diagnostics, change various parameters,increase/decrease the desired stimulus amplitude of the electrodepulses, and adjust feedback sensitivity of the RF pulse generator module106, among other functions.

The RF pulse generator module 106 may include communication electronicsthat support the wireless connection 104, the stimulation circuitry, andthe battery to power the generator electronics. In some implementations,the RF pulse generator module 106 includes the TX antenna embedded intoits packaging form factor while, in other implementations, the TXantenna is connected to the RF pulse generator module 106 through awired connection 108 or a wireless connection (not shown). The TXantenna 110 may be coupled directly to tissue to create an electricfield that powers the implanted wireless stimulator device 114. The TXantenna 110 communicates with the implanted wireless stimulator device114 through an RF interface. For instance, the TX antenna 110 radiatesan RF transmission signal that is modulated and encoded by the RF pulsegenerator module 110. The implanted wireless stimulator device of module114 contains one or more antennas, such as dipole antenna(s), to receiveand transmit through RF interface 112. In particular, the couplingmechanism between antenna 110 and the one or more antennas on theimplanted wireless stimulation device of module 114 utilizes electricalradiative coupling and not inductive coupling. In other words, thecoupling is through an electric field rather than a magnetic field.

Through this electrical radiative coupling, the TX antenna 110 canprovide an input signal to the implanted wireless stimulator device 114.This input signal contains energy and may contain information encodingstimulus waveforms to be applied at the electrodes of the implantedwireless stimulator device 114. In some implementations, the power levelof this input signal directly determines an applied amplitude (forexample, power, current, or voltage) of the one or more electricalpulses created using the electrical energy contained in the inputsignal. Within the implanted wireless stimulator device 114 arecomponents for demodulating the RF transmission signal, and electrodesto deliver the stimulation to surrounding neuronal tissue.

The RF pulse generator module 106 can be implanted subcutaneously, or itcan be worn external to the body. When external to the body, the RFgenerator module 106 can be incorporated into a belt or harness designto allow for electric radiative coupling through the skin and underlyingtissue to transfer power and/or control parameters to the implantedwireless stimulator device 114. In either event, receiver circuit(s)internal to the wireless stimulator device 114 (or cylindrical wirelessimplantable stimulator device 1400 shown in FIGS. 14A and 14B, helicalwireless implantable stimulator device 1900 shown in FIGS. 19A to 19C)can capture the energy radiated by the TX antenna 110 and convert thisenergy to an electrical waveform. The receiver circuit(s) may furthermodify the waveform to create an electrical pulse suitable for thestimulation of neural tissue.

In some implementations, the RF pulse generator module 106 can remotelycontrol the stimulus parameters (that is, the parameters of theelectrical pulses applied to the neural tissue) and monitor feedbackfrom the wireless stimulator device 114 based on RF signals receivedfrom the implanted wireless stimulator device 114. A feedback detectionalgorithm implemented by the RF pulse generator module 106 can monitordata sent wirelessly from the implanted wireless stimulator device 114,including information about the energy that the implanted wirelessstimulator device 114 is receiving from the RF pulse generator andinformation about the stimulus waveform being delivered to the electrodepads. In order to provide an effective therapy for a given medicalcondition, the system can be tuned to provide the optimal amount ofexcitation or inhibition to the nerve fibers by electrical stimulation.A closed loop feedback control method can be used in which the outputsignals from the implanted wireless stimulator device 114 are monitoredand used to determine the appropriate level of neural stimulationcurrent for maintaining effective neuronal activation, or, in somecases, the patient can manually adjust the output signals in an openloop control method.

FIG. 2 depicts a detailed diagram of an example of the wirelessstimulation system. As depicted, the programming module 102 may compriseuser input system 202 and communication subsystem 208. The user inputsystem 221 may allow various parameter settings to be adjusted (in somecases, in an open loop fashion) by the user in the form of instructionsets. The communication subsystem 208 may transmit these instructionsets (and other information) via the wireless connection 104, such asBluetooth or Wi-Fi, to the RF pulse generator module 106, as well asreceive data from module 106.

For instance, the programmer module 102, which can be utilized formultiple users, such as a patient's control unit or clinician'sprogrammer unit, can be used to send stimulation parameters to the RFpulse generator module 106. The stimulation parameters that can becontrolled may include pulse amplitude, pulse frequency, and pulse widthin the ranges shown in Table 1. In this context the term pulse refers tothe phase of the waveform that directly produces stimulation of thetissue; the parameters of the charge-balancing phase (described below)can similarly be controlled. The patient and/or the clinician can alsooptionally control overall duration and pattern of treatment.

TABLE 1 Stimulation Parameter Pulse Amplitude: 0 to 25 mA PulseFrequency: 0 to 20000 Hz Pulse Width: 0 to 2 ms

The RF pulse generator module 106 may be initially programmed to meetthe specific parameter settings for each individual patient during theinitial implantation procedure. Because medical conditions or the bodyitself can change over time, the ability to re-adjust the parametersettings may be beneficial to ensure ongoing efficacy of the neuralmodulation therapy.

The programmer module 102 may be functionally a smart device andassociated application. The smart device hardware may include a CPU 206and be used as a vehicle to handle touchscreen input on a graphical userinterface (GUI) 204, for processing and storing data.

The RF pulse generator module 106 may be connected via wired connection108 to an external TX antenna 110. Alternatively, both the antenna andthe RF pulse generator are located subcutaneously (not shown).

The signals sent by RF pulse generator module 106 to the implantedwireless stimulator device 114 may include both power andparameter-setting attributes in regards to stimulus waveform, amplitude,pulse width, and frequency. The RF pulse generator module 106 can alsofunction as a wireless receiving unit that receives feedback signalsfrom the implanted wireless stimulator device 114. To that end, the RFpulse generator module 106 may contain microelectronics or othercircuitry to handle the generation of the signals transmitted to thedevice 114 as well as handle feedback signals, such as those from thestimulator device 114. For example, the RF pulse generator module 106may comprise controller subsystem 214, high-frequency oscillator 218, RFamplifier 216, a RF switch, and a feedback subsystem 212.

The controller subsystem 214 may include a CPU 230 to handle dataprocessing, a memory subsystem 228 such as a local memory, communicationsubsystem 234 to communicate with programmer module 102 (includingreceiving stimulation parameters from programmer module), pulsegenerator circuitry 236, and digital/analog (D/A) converters 232.

The controller subsystem 214 may be used by the patient and/or theclinician to control the stimulation parameter settings (for example, bycontrolling the parameters of the signal sent from RF pulse generatormodule 106 to the stimulator device 114). These parameter settings canaffect, for example, the power, current level, or shape of the one ormore electrical pulses. The programming of the stimulation parameterscan be performed using the programming module 102, as described above,to set the repetition rate, pulse width, amplitude, and waveform thatwill be transmitted by RF energy to the receiving (RX) antenna 238,typically a dipole antenna (although other types may be used), in theimplanted wireless stimulation device 214. The clinician may have theoption of locking and/or hiding certain settings within the programmerinterface, thus limiting the patient's ability to view or adjust certainparameters because adjustment of certain parameters may require detailedmedical knowledge of neurophysiology, neuro-anatomy, protocols forneural modulation, and safety limits of electrical stimulation.

The controller subsystem 214 may store received parameter settings inthe local memory subsystem 228, until the parameter settings aremodified by new input data received from the programming module 102. TheCPU 206 may use the parameters stored in the local memory to control thepulse generator circuitry 236 to generate a stimulus waveform that ismodulated by a high frequency oscillator 218 in the range from 300 MHzto 8 GHz (preferably between about 700 MHz and 5.8 GHz and morepreferably between about 800 MHz and 1.3 GHz). The resulting RF signalmay then be amplified by RF amplifier 226 and then sent through an RFswitch 223 to the TX antenna 110 to reach through depths of tissue tothe RX antenna 238.

In some implementations, the RF signal sent by TX antenna 110 may simplybe a power transmission signal used by the wireless stimulation devicemodule 114 to generate electric pulses. In other implementations, atelemetry signal may also be transmitted to the wireless stimulatordevice 114 to send instructions about the various operations of thewireless stimulator device 114. The telemetry signal may be sent by themodulation of the carrier signal (through the skin if external, orthrough other body tissues if the pulse generator module 106 isimplanted subcutaneously). The telemetry signal is used to modulate thecarrier signal (a high frequency signal) that is coupled onto theimplanted antenna(s) 238 and does not interfere with the input receivedon the same stimulator device to power the device. In one embodiment thetelemetry signal and powering signal are combined into one signal, wherethe RF telemetry signal is used to modulate the RF powering signal, andthus the wireless stimulation device is powered directly by the receivedtelemetry signal; separate subsystems in the wireless stimulation deviceharness the power contained in the signal and interpret the data contentof the signal.

The RF switch 223 may be a multipurpose device such as a dualdirectional coupler, which passes the relatively high amplitude,extremely short duration RF pulse to the TX antenna 110 with minimalinsertion loss while simultaneously providing two low-level outputs tofeedback subsystem 212; one output delivers a forward power signal tothe feedback subsystem 212, where the forward power signal is anattenuated version of the RF pulse sent to the TX antenna 110, and theother output delivers a reverse power signal to a different port of thefeedback subsystem 212, where reverse power is an attenuated version ofthe reflected RF energy from the TX Antenna 110.

During the on-cycle time (when an RF signal is being transmitted towireless stimulator device 114), the RF switch 223 is set to send theforward power signal to feedback subsystem. During the off-cycle time(when an RF signal is not being transmitted to the wireless stimulatordevice 114), the RF switch 223 can change to a receiving mode in whichthe reflected RF energy and/or RF signals from the wireless stimulatordevice 114 are received to be analyzed in the feedback subsystem 212.

The feedback subsystem 212 of the RF pulse generator module 106 mayinclude reception circuitry to receive and extract telemetry or otherfeedback signals from the wireless stimulator device 114 and/orreflected RF energy from the signal sent by TX antenna 110. The feedbacksubsystem may include an amplifier 226, a filter 224, a demodulator 222,and an A/D converter 220.

The feedback subsystem 212 receives the forward power signal andconverts this high-frequency AC signal to a DC level that can be sampledand sent to the controller subsystem 214. In this way thecharacteristics of the generated RF pulse can be compared to a referencesignal within the controller subsystem 214. If a disparity (error)exists in any parameter, the controller subsystem 214 can adjust theoutput to the RF pulse generator 106. The nature of the adjustment canbe, for example, proportional to the computed error. The controllersubsystem 214 can incorporate additional inputs and limits on itsadjustment scheme such as the signal amplitude of the reverse power andany predetermined maximum or minimum values for various pulseparameters.

The reverse power signal can be used to detect fault conditions in theRF-power delivery system. In an ideal condition, when TX antenna 110 hasperfectly matched impedance to the tissue that it contacts, theelectromagnetic waves generated from the RF pulse generator 106 passunimpeded from the TX antenna 110 into the body tissue. However, inreal-world applications a large degree of variability may exist in thebody types of users, types of clothing worn, and positioning of theantenna 110 relative to the body surface. Since the impedance of theantenna 110 depends on the relative permittivity of the underlyingtissue and any intervening materials, and also depends on the overallseparation distance of the antenna from the skin, in any givenapplication there can be an impedance mismatch at the interface of theTX antenna 110 with the body surface. When such a mismatch occurs, theelectromagnetic waves sent from the RF pulse generator 106 are partiallyreflected at this interface, and this reflected energy propagatesbackward through the antenna feed.

The dual directional coupler RF switch 223 may prevent the reflected RFenergy propagating back into the amplifier 226, and may attenuate thisreflected RF signal and send the attenuated signal as the reverse powersignal to the feedback subsystem 212. The feedback subsystem 212 canconvert this high-frequency AC signal to a DC level that can be sampledand sent to the controller subsystem 214. The controller subsystem 214can then calculate the ratio of the amplitude of the reverse powersignal to the amplitude of the forward power signal. The ratio of theamplitude of reverse power signal to the amplitude level of forwardpower may indicate severity of the impedance mismatch.

In order to sense impedance mismatch conditions, the controllersubsystem 214 can measure the reflected-power ratio in real time, andaccording to preset thresholds for this measurement, the controllersubsystem 214 can modify the level of RF power generated by the RF pulsegenerator 106. For example, for a moderate degree of reflected power thecourse of action can be for the controller subsystem 214 to increase theamplitude of RF power sent to the TX antenna 110, as would be needed tocompensate for slightly non-optimum but acceptable TX antenna couplingto the body. For higher ratios of reflected power, the course of actioncan be to prevent operation of the RF pulse generator 106 and set afault code to indicate that the TX antenna 110 has little or no couplingwith the body. This type of reflected-power fault condition can also begenerated by a poor or broken connection to the TX antenna. In eithercase, it may be desirable to stop RF transmission when thereflected-power ratio is above a defined threshold, because internallyreflected power can result in unwanted heating of internal components,and this fault condition means the system cannot deliver sufficientpower to the implanted wireless stimulation device and thus cannotdeliver therapy to the user.

The controller 242 of the wireless stimulator device 114 may transmitinformational signals, such as a telemetry signal, through the antenna238 to communicate with the RF pulse generator module 106 during itsreceive cycle. For example, the telemetry signal from the wirelessstimulator device 114 may be coupled to the modulated signal on thedipole antenna(s) 238, during the on and off state of the transistorcircuit to enable or disable a waveform that produces the correspondingRF bursts necessary to transmit to the external (or remotely implanted)pulse generator module 106. The antenna(s) 238 may be connected toelectrodes 254 in contact with tissue to provide a return path for thetransmitted signal. An A/D (not shown) converter can be used to transferstored data to a serialized pattern that can be transmitted on thepulse-modulated signal from the internal antenna(s) 238 of the wirelessstimulator device 114.

A telemetry signal from the implanted wireless stimulator device 114 mayinclude stimulus parameters such as the power or the amplitude of thecurrent that is delivered to the tissue from the electrodes. Thefeedback signal can be transmitted to the RF pulse generator module 116to indicate the strength of the stimulus at the nerve bundle by means ofcoupling the signal to the implanted RX antenna 238, which radiates thetelemetry signal to the external (or remotely implanted) RF pulsegenerator module 106. The feedback signal can include either or both ananalog and digital telemetry pulse modulated carrier signal. Data suchas stimulation pulse parameters and measured characteristics ofstimulator performance can be stored in an internal memory device withinthe implanted stimulator device 114, and sent on the telemetry signal.The frequency of the carrier signal may be in the range of at 300 MHz to8 GHz (preferably between about 700 MHz and 5.8 GHz and more preferablybetween about 800 MHz and 1.3 GHz).

In the feedback subsystem 212, the telemetry signal can be downmodulated using demodulator 222 and digitized by being processed throughan analog to digital (A/D) converter 220. The digital telemetry signalmay then be routed to a CPU 230 with embedded code, with the option toreprogram, to translate the signal into a corresponding currentmeasurement in the tissue based on the amplitude of the received signal.The CPU 230 of the controller subsystem 214 can compare the reportedstimulus parameters to those held in local memory 228 to verify thewireless stimulator device 114 delivered the specified stimuli totissue. For example, if the wireless stimulation device reports a lowercurrent than was specified, the power level from the RF pulse generatormodule 106 can be increased so that the implanted wireless stimulatordevice 114 will have more available power for stimulation. The implantedwireless stimulator device 114 can generate telemetry data in real time,for example, at a rate of 8 Kbits per second. All feedback data receivedfrom the implanted stimulator device 114 can be logged against time andsampled to be stored for retrieval to a remote monitoring systemaccessible by the health care professional for trending and statisticalcorrelations.

The sequence of remotely programmable RF signals received by theinternal antenna(s) 238 may be conditioned into waveforms that arecontrolled within the implantable wireless stimulator device 114 by thecontrol subsystem 242 and routed to the appropriate electrodes 254 thatare placed in proximity to the tissue to be stimulated. For instance,the RF signal transmitted from the RF pulse generator module 106 may bereceived by RX antenna 238 and processed by circuitry, such as waveformconditioning circuitry 240, within the implanted wireless stimulatordevice 114 to be converted into electrical pulses applied to theelectrodes 254 through electrode interface 252. In some implementations,the implanted wireless stimulator device 114 contains between two tosixteen electrodes 254.

The waveform conditioning circuitry 240 may include a rectifier 244,which rectifies the signal received by the RX antenna 238. The rectifiedsignal may be fed to the controller 242 for receiving encodedinstructions from the RF pulse generator module 106. The rectifiersignal may also be fed to a charge balance component 246 that isconfigured to create one or more electrical pulses based such that theone or more electrical pulses result in a substantially zero net chargeat the one or more electrodes (that is, the pulses are charge balanced).The charge-balanced pulses are passed through the current limiter 248 tothe electrode interface 252, which applies the pulses to the electrodes254 as appropriate.

The current limiter 248 insures the current level of the pulses appliedto the electrodes 254 is not above a threshold current level. In someimplementations, an amplitude (for example, current level, voltagelevel, or power level) of the received RF pulse directly determines theamplitude of the stimulus. In this case, it may be particularlybeneficial to include current limiter 248 to prevent excessive currentor charge being delivered through the electrodes, although currentlimiter 248 may be used in other implementations where this is not thecase. Generally, for a given electrode having several square millimeterssurface area, it is the charge per phase that should be limited forsafety (where the charge delivered by a stimulus phase is the integralof the current). But, in some cases, the limit can instead be placed onthe current, where the maximum current multiplied by the maximumpossible pulse duration is less than or equal to the maximum safecharge. More generally, the limiter 248 acts as a charge limiter thatlimits a characteristic (for example, current or duration) of theelectrical pulses so that the charge per phase remains below a thresholdlevel (typically, a safe-charge limit).

In the event the implanted wireless stimulator device 114 receives a“strong” pulse of RF power sufficient to generate a stimulus that wouldexceed the predetermined safe-charge limit, the current limiter 248 canautomatically limit or “clip” the stimulus phase to maintain the totalcharge of the phase within the safety limit. The current limiter 248 maybe a passive current limiting component that cuts the signal to theelectrodes 254 once the safe current limit (the threshold current level)is reached. Alternatively, or additionally, the current limiter 248 maycommunicate with the electrode interface 252 to turn off all electrodes254 to prevent tissue damaging current levels.

A clipping event may trigger a current limiter feedback control mode.The action of clipping may cause the controller to send a thresholdpower data signal to the pulse generator 106. The feedback subsystem 212detects the threshold power signal and demodulates the signal into datathat is communicated to the controller subsystem 214. The controllersubsystem 214 algorithms may act on this current-limiting condition byspecifically reducing the RF power generated by the RF pulse generator,or cutting the power completely. In this way, the pulse generator 106can reduce the RF power delivered to the body if the implanted wirelessstimulator device 114 reports it is receiving excess RF power.

The controller 250 of the stimulator 205 may communicate with theelectrode interface 252 to control various aspects of the electrodesetup and pulses applied to the electrodes 254. The electrode interface252 may act as a multiplex and control the polarity and switching ofeach of the electrodes 254. For instance, in some implementations, thewireless stimulator 106 has multiple electrodes 254 in contact withtissue, and for a given stimulus the RF pulse generator module 106 canarbitrarily assign one or more electrodes to 1) act as a stimulatingelectrode, 2) act as a return electrode, or 3) be inactive bycommunication of assignment sent wirelessly with the parameterinstructions, which the controller 250 uses to set electrode interface252 as appropriate. It may be physiologically advantageous to assign,for example, one or two electrodes as stimulating electrodes and toassign all remaining electrodes as return electrodes.

Also, in some implementations, for a given stimulus pulse, thecontroller 250 may control the electrode interface 252 to divide thecurrent arbitrarily (or according to instructions from pulse generatormodule 106) among the designated stimulating electrodes. This controlover electrode assignment and current control can be advantageousbecause in practice the electrodes 254 may be spatially distributedalong various neural structures, and through strategic selection of thestimulating electrode location and the proportion of current specifiedfor each location, the aggregate current distribution in tissue can bemodified to selectively activate specific neural targets. This strategyof current steering can improve the therapeutic effect for the patient.

In another implementation, the time course of stimuli may be arbitrarilymanipulated. A given stimulus waveform may be initiated at a timeT_start and terminated at a time T_final, and this time course may besynchronized across all stimulating and return electrodes; further, thefrequency of repetition of this stimulus cycle may be synchronous forall the electrodes. However, controller 250, on its own or in responseto instructions from pulse generator 106, can control electrodeinterface 252 to designate one or more subsets of electrodes to deliverstimulus waveforms with non-synchronous start and stop times, and thefrequency of repetition of each stimulus cycle can be arbitrarily andindependently specified.

For example, a stimulator having eight electrodes may be configured tohave a subset of five electrodes, called set A, and a subset of threeelectrodes, called set B. Set A might be configured to use two of itselectrodes as stimulating electrodes, with the remainder being returnelectrodes. Set B might be configured to have just one stimulatingelectrode. The controller 250 could then specify that set A delivered astimulus phase, for example, 3 mA current for a duration of 200microseconds followed by a 400 microseconds charge-balancing phase. Thisstimulus cycle, for example, could be specified to repeat at a rate of60 cycles per second. Then, for set B, the controller 250 could specifya stimulus phase of 1 mA current for duration of 500 microseconds,followed by a 800 microseconds charge-balancing phase. The repetitionrate for the set-B stimulus cycle can be set independently of set A, sayfor example, could be specified at 25 cycles per second. Or, if thecontroller 250 was configured to match the repetition rate for set B tothat of set A, for such a case, the controller 250 can specify therelative start times of the stimulus cycles to be coincident in time orto be arbitrarily offset from one another by some delay interval.

In some implementations, the controller 250 can arbitrarily shape thestimulus waveform amplitude, and may do so in response to instructionsfrom pulse generator 106. The stimulus phase may be delivered by aconstant-current source or a constant-voltage source, and this type ofcontrol may generate characteristic waveforms that are static, e.g. aconstant-current source generates a characteristic rectangular pulse inwhich the current waveform has a very steep rise, a constant amplitudefor the duration of the stimulus, and then a very steep return tobaseline. Alternatively, or additionally, the controller 250 canincrease or decrease the level of current at any time during thestimulus phase and/or during the charge-balancing phase. Thus, in someimplementations, the controller 250 can deliver arbitrarily shapedstimulus waveforms such as a triangular pulse, sinusoidal pulse, orGaussian pulse for example. Similarly, the charge-balancing phase can bearbitrarily amplitude-shaped, and similarly a leading anodic pulse(prior to the stimulus phase) may also be amplitude-shaped.

As described above, the wireless stimulator device 114 may include acharge-balancing component 246. Generally, for constant currentstimulation pulses, pulses should be charge balanced by having theamount of cathodic current should equal the amount of anodic current,which is typically called biphasic stimulation. Charge density is theamount of current times the duration it is applied, and is typicallyexpressed in the units uC/cm². In order to avoid the irreversibleelectrochemical reactions such as pH change, electrode dissolution aswell as tissue destruction, no net charge should appear at theelectrode-electrolyte interface, and it is generally acceptable to havea charge density less than 30 uC/cm². Biphasic stimulating currentpulses ensure that no net charge appears at the electrode after eachstimulation cycle and the electrochemical processes are balanced toprevent net dc currents. The wireless stimulator device 114 may bedesigned to ensure that the resulting stimulus waveform has a net zerocharge. Charge balanced stimuli are thought to have minimal damagingeffects on tissue by reducing or eliminating electrochemical reactionproducts created at the electrode-tissue interface.

A stimulus pulse may have a negative-voltage or current, called thecathodic phase of the waveform. Stimulating electrodes may have bothcathodic and anodic phases at different times during the stimulus cycle.An electrode that delivers a negative current with sufficient amplitudeto stimulate adjacent neural tissue is called a “stimulating electrode.”During the stimulus phase the stimulating electrode acts as a currentsink. One or more additional electrodes act as a current source andthese electrodes are called “return electrodes.” Return electrodes areplaced elsewhere in the tissue at some distance from the stimulatingelectrodes. When a typical negative stimulus phase is delivered totissue at the stimulating electrode, the return electrode has a positivestimulus phase. During the subsequent charge-balancing phase, thepolarities of each electrode are reversed.

In some implementations, the charge balance component 246 uses ablocking capacitor(s) placed electrically in series with the stimulatingelectrodes and body tissue, between the point of stimulus generationwithin the stimulator circuitry and the point of stimulus delivery totissue. In this manner, a resistor-capacitor (RC) network may be formed.In a multi-electrode stimulator, one charge-balance capacitor(s) may beused for each electrode or a centralized capacitor(s) may be used withinthe stimulator circuitry prior to the point of electrode selection. TheRC network can block direct current (DC), however it can also preventlow-frequency alternating current (AC) from passing to the tissue. Thefrequency below which the series RC network essentially blocks signalsis commonly referred to as the cutoff frequency, and in one embodimentthe design of the stimulator system may ensure the cutoff frequency isnot above the fundamental frequency of the stimulus waveform. In thisembodiment as disclosed herein, the wireless stimulator may have acharge-balance capacitor with a value chosen according to the measuredseries resistance of the electrodes and the tissue environment in whichthe stimulator is implanted. By selecting a specific capacitance valuethe cutoff frequency of the RC network in this embodiment is at or belowthe fundamental frequency of the stimulus pulse.

In other implementations, the cutoff frequency may be chosen to be at orabove the fundamental frequency of the stimulus, and in this scenariothe stimulus waveform created prior to the charge-balance capacitor,called the drive waveform, may be designed to be non-stationary, wherethe envelope of the drive waveform is varied during the duration of thedrive pulse. For example, in one embodiment, the initial amplitude ofthe drive waveform is set at an initial amplitude Vi, and the amplitudeis increased during the duration of the pulse until it reaches a finalvalue k*Vi. By changing the amplitude of the drive waveform over time,the shape of the stimulus waveform passed through the charge-balancecapacitor is also modified. The shape of the stimulus waveform may bemodified in this fashion to create a physiologically advantageousstimulus.

In some implementations, the wireless stimulator device 114 may create adrive-waveform envelope that follows the envelope of the RF pulsereceived by the receiving dipole antenna(s) 238. In this case, the RFpulse generator module 106 can directly control the envelope of thedrive waveform within the wireless stimulator device 114, and thus noenergy storage may be required inside the stimulator itself. In thisimplementation, the stimulator circuitry may modify the envelope of thedrive waveform or may pass it directly to the charge-balance capacitorand/or electrode-selection stage.

In some implementations, the implanted wireless stimulator device 114may deliver a single-phase drive waveform to the charge balancecapacitor or it may deliver multiphase drive waveforms. In the case of asingle-phase drive waveform, for example, a negative-going rectangularpulse, this pulse comprises the physiological stimulus phase, and thecharge-balance capacitor is polarized (charged) during this phase. Afterthe drive pulse is completed, the charge balancing function is performedsolely by the passive discharge of the charge-balance capacitor, whereis dissipates its charge through the tissue in an opposite polarityrelative to the preceding stimulus. In one implementation, a resistorwithin the stimulator facilitates the discharge of the charge-balancecapacitor. In some implementations, using a passive discharge phase, thecapacitor may allow virtually complete discharge prior to the onset ofthe subsequent stimulus pulse.

In the case of multiphase drive waveforms the wireless stimulator mayperform internal switching to pass negative-going or positive-goingpulses (phases) to the charge-balance capacitor. These pulses may bedelivered in any sequence and with varying amplitudes and waveformshapes to achieve a desired physiological effect. For example, thestimulus phase may be followed by an actively driven charge-balancingphase, and/or the stimulus phase may be preceded by an opposite phase.Preceding the stimulus with an opposite-polarity phase, for example, canhave the advantage of reducing the amplitude of the stimulus phaserequired to excite tissue.

In some implementations, the amplitude and timing of stimulus andcharge-balancing phases is controlled by the amplitude and timing of RFpulses from the RF pulse generator module 106, and in others thiscontrol may be administered internally by circuitry onboard the wirelessstimulator device 114, such as controller 250. In the case of onboardcontrol, the amplitude and timing may be specified or modified by datacommands delivered from the pulse generator module 106.

FIG. 3 illustrates a transmitting antenna 302 coupling electromagneticenergy to a receiving dipole antenna 306 through a subject's skin 304.In particular, FIG. 3 illustrates the transmitting antenna 302 and itsplacement when the transmitting antenna 302 is transmitting to areceiving dipole antenna 306 of a stimulator device that is implantedunder the skin 304. In this disclosure, transmitting antenna 302 refersto the assembly of components on a device that interact with anout-bound electromagnetic wave. In this example, the receiving dipoleantenna 306 is part of a stimulator device implanted close to spinalcord 308 for stimulation thereof. Configurations may vary, for example,in separation distance from the transmitting antenna 302 to the skin 304as well as the depth of the receiving dipole antenna 306 into the humanbody. In some configurations, the transmitting antenna 302 may beseparated from the skin 304 by a distance of as short as 2 mm. In someexamples, the depth of the receiving dipole antenna 306 may be up to 6cm below the skin 304. Transmitting antenna 202 can accommodate forvariances in separation distances when transmitting antenna 302 isjudiciously configured to mitigate attenuation caused by longerdistances. Such configurations may generally include size, shape, andpolarization configurations. These configurations may include thin andflexible antenna assemblies. For example, the antenna assembly may beless than 200 μm in thickness. The antenna assembly may be bent up to 50degrees while remaining operational in that the reflection ratio ismaintained at 6 dB or better at the operating frequency of the antennaassembly.

In some instances, simulations may be performed to model the coupling ofelectromagnetic energy from the transmitting antenna assembly to areceiving dipole, for example, via radiative coupling. In one suchexample, as illustrated in FIG. 3 the antenna is located in proximity tothe spinal cord centered in the vertebrae at L1. Simulations may be runwith an ANSYS HFSS human body model. The receiving dipole antenna 306,realistically curved, is highlighted. Some implementations allow forthin and flexible transmitting antenna configurations that suit theoverall ergonomics as a wearable medical device. For example, theantenna configurations of these implementations are comfortable to wearand easy to conceal when worn by a subject.

As discussed below, in some implementations, the transmitting antenna302 is configured as a wide dipole antenna assembly while in some otherimplementations the antenna assembly is configured as bowtie antennaassembly. In either case, the antenna assembly may be configured to emitthe linearly polarized electromagnetic energy efficiently. For example,the antenna assembly may operate with a reflection ratio of at least 6dB at an operating frequency of the antenna assembly. The reflectionratio corresponds to a ratio of a transmission power used by the antennaassembly to transmit the input signal and a reflection power seen by theantenna assembly resulting from electromagnetic emission of the inputsignal. The reflection ratio of at least 6 dB may be maintainedregardless of a separation between the metal layer and the subject'sskin. Notably, the antenna assembly may be configured to emit, withoutdirect contact with the subject's skin, the linearly polarizedelectromagnetic energy to the receiving dipole antenna. The emission isaccomplished with an air gap between the antenna assembly and thesubject's skin and without gel coupling between the metal layer and thesubject's skin. The efficient transmission may have a broadbandcharacteristic in that the antenna assembly is configured to operatewith a quality factor (Q) no more than 9.

FIGS. 4A-4B show an example of transmitting antenna 302 configured as awide dipole antenna assembly 400 for operation at 915 MHz. Asillustrated, wide dipole antenna assembly 400 is generally rectangularin shape with a length 406 in the range of 4 cm to 20 cm, a width 404 of1 cm to 5 cm and four rounded corners or fillets 408A to 408B. Theparticular example shown has a length of 12 cm and a width of 2.54 cm.As shown in FIG. 4B, the wide dipole antenna assembly 400 includes aninward surface 400A to radiate EM energy towards the implantedstimulator device underneath the skin. Inward surface 400A may be placedin close proximity to the skin surface of a human patient. Outwardsurface 400B may provide protection against mechanical wear and tear.FIG. 4B further shows 50Ω feed port 412, for connecting wide dipoleantenna assembly 400 to, for example, a signal generator such as RFpulse generator module 106. This 50Ω feed port 412 may be at the midlineof the outward surface 400B. By way of example, BNC (BonetNeill-Concelman) or SMC (SubMiniature version A) type connectors can beused to connect 50Ω feed port 412 to an MFS device through a co-axialcable.

FIGS. 4A and 4B also show a receiving dipole antenna 402 relative to thewide dipole antenna assembly 400. As illustrated, the wide dipoleantenna assembly 400 can be located 1 cm to 3 cm (410) from thereceiving antenna 402. Wide dipole antenna assembly 400 radiates EMenergy into the human body via the two ends along a longitudinal axis ofmetal layer. In particular, the wide dipole antenna 400 has one endformed by corners 408A and 408B, and another end formed by corners 408Cand 408D. The two ends may form a dipole configuration capable oftransmitting electromagnetic waves linearly polarized along thedirection of the two ends. When coupling to the receiving dipole antenna402, this linear polarization of antenna assembly is aligned with thelong axis of receiving dipole antenna 402. In some configurations, themetal layer is as small as 20 to 200 μm in thickness.

FIG. 5 shows example S-parameter results of the wide dipole antennaassembly 400. Here, the S11 parameter (input port voltage reflectioncoefficient) and S22 parameter (output port voltage reflectioncoefficient) both have notches at the operating frequency of 915 MHz.Moreover, the S22 parameter is approximately 8 dB or so lower than theS11 parameter. These resonance performances are achieved withoutcompromising the S21 parameter, which is the forward voltage gain.Notably, as shown in FIG. 9 , the S11 parameter is under −10 dB at theoperating frequency and, more specifically, is approximately −15.5 dB atthe operating frequency. Accordingly, the wide dipole antenna assembly400 is configured to operate with a reflection ratio of at least 10 dB.The reflection ratio corresponds to a ratio of the transmission powerused by the wide dipole antenna assembly to emit the linearly polarizedelectromagnetic energy and the reflection power seen by the wide dipoleantenna assembly resulting from electromagnetic emission using thetransmission power. The transmission power represents the power levelused by the transmitting antenna—wide dipole antenna assembly 400—inemitting linearly polarized electromagnetic energy so that the inputsignal containing electrical energy is sent to the receiving antenna onthe implantable stimulator device. Meanwhile, the reflection powerrefers to the reflected power back to the RF source from theantenna—wide dipole antenna assembly 400. A 15 dB or more suppressionmeans about 3% of the transmitted energy may get reflected. In otherwords, about 97% of the transmitted energy passes through. The antennaassembly 400 may be configured such that the reflection ratio of atleast 10 dB at the operating frequency of the antenna assembly ismaintained when the antenna assembly is positioned between zero to 2centimeters, or more particularly, between zero to 1 centimeter, awayfrom the patient's skin. That is, the antenna assembly 400 may beconfigured such that the S11 parameter notch at the operating frequencyis wide enough that the S11 parameter remains below −10 dB as theantenna assembly 400 is positioned between 0 to 2 centimeters, or moreparticularly, between zero to 1 centimeter, away from the patient'sskin.

FIG. 6A-6B shows an example radiation patterns of the wide dipoleantenna assembly. FIG. 6A shows the specific absorption rate (SAR) fieldpattern in a plane parallel to the length direction of wide dipoleantenna assembly 400 for an average input power of 0.2 W and at 915 MHz.FIG. 6B shows the same specific absorption rate (SAR) field pattern in aplane perpendicular to the length direction of wide dipole antennaassembly 400 for an average input power of 0.2 W and at 915 MHz. In thisexample, the radiation patterns demonstrate that the field coverageextends sufficiently into the human body at various tissue depths withless than 25% reduction in field strength.

FIGS. 7A-7B shows an example of a transmitting antenna configured as awide dipole antenna assembly 700 for operation at 2.4 GHz. FIG. 7Adepicts a top view of the wide dipole antenna assembly 700, which haslength in the range of 4 cm to 20 cm, a width 404 of 1 cm to 5 cm andfour rounded corners or fillets. The particular example shown has alength 706 of 5 cm, a width 704 of 2.54 cm and four rounded fillets 708Ato 708D, each rounded with a radius of 6.35 mm. The rounding maycontribute to patient safety in that the round corners mitigateaccidental injury due to sharp corners. As shown in FIG. 7B, wide dipoleantenna assembly 700 includes an inward surface 700A to radiate EMenergy towards the implanted stimulator device underneath the skin.Inward surface 700A may include signal metal layer. Inward surface 700Amay be placed in proximity of the skin surface of a patient. Theplacement may be without gel coupling between the skin surface and thesignal metal layer. Wide dipole antenna assembly 700 also includes anoutward surface 700B which may be placed to face away from the skin.Outward surface 700B may provide protection against mechanical wear andtear. FIG. 7B further shows 50Ω feed port 712, for connecting widedipole antenna assembly 700 to, for example, a microwave fieldstimulator (MFS) device. This 50Ω feed port 712 may be along the midlineof the outward surface 700B. By way of example, BNC (BonetNeill-Concelman) or SMC (SubMiniature version A) type connectors can beused to connect 50Ω feed port 712 to an MFS device through a co-axialcable.

FIGS. 7A and 7B also show receiving dipole 702 relative to the widedipole antenna assembly 700. As illustrated, wide dipole patch antennaassembly 700 can be located 1.1 cm (710) from the receiving antenna 702.Wide dipole antenna assembly 700 radiates EM energy into the human bodyvia the two ends along a longitudinal axis of metal layer. Inparticular, the wide dipole antenna assembly 700 has one end formed bycorners 708A and 708B, and another end formed by corners 708C and 708D.The two ends may form a dipole configuration capable of transmittingelectromagnetic waves linearly polarized along the direction of the twoends. When coupling to the receiving dipole antenna 702, this linearpolarization of antenna assembly is aligned with the long axis ofreceiving dipole antenna 702. In some configurations, the metal layer isas small as 20 to 200 μm in thickness.

FIG. 8 shows S-parameter results of the wide dipole antenna assembly.Here, the S11 parameter (input port voltage reflection coefficient) andS22 parameter (output port voltage reflection coefficient) both havenotches at the operating frequency of 2.4 GHz. Moreover, the S22parameter is about 17 dB or so lower than the S11 parameter. Theseresonance performances are achieved without compromising the S21parameter, which is the forward voltage gain. Notably, the S11 parameterindicates a reflection ratio of at least 6 dB at the operating frequencyand, in particular, more than 6.5 dB. The antenna assembly 700 may beconfigured such that the reflection ratio of at least 6 dB at theoperating frequency of the antenna assembly is maintained when theantenna assembly is positioned between zero to 2 centimeters, or moreparticularly, between zero to 1 centimeter, away from the patient'sskin. That is, the antenna assembly 700 may be configured such that theS11 parameter notch at the operating frequency is wide enough that theS11 parameter remains below −6 dB as the antenna assembly 700 ispositioned between 0 to 2 centimeters, or more particularly, betweenzero to 1 centimeter, away from the patient's skin.

FIG. 9 shows example radiation patterns of the wide dipole antennaassembly. In particular, FIG. 9 shows the specific absorption rate (SAR)field pattern in a plane parallel to the length direction of wide dipoleantenna assembly 700 for an average input power of 0.2 W and at 2.4 GHz.The specific absorption rate (SAR) field pattern in the planeperpendicular to the length direction of wide dipole antenna assembly700 has similar pattern. Here in this example, the radiation patternsdemonstrate that the field coverage extends sufficiently into the humanbody at various tissue depths with less than 25% reduction in fieldstrength.

FIGS. 10A-10C show an example of a transmitting antenna configured as abowtie antenna assembly 1000. For context, the illustrations in FIGS.10A-10C show the mesh grid of a finite element model used in simulationexperiments to investigate performance of bowtie antenna assembly 1000.In particular, FIG. 10A shows an example bowtie antenna assembly 1000with a coax feed 1002 for a coax connection to a controller device suchas a microwave field stimulator. The coax feed generally assumes a 50Ωload. The bowtie antenna assembly 1000, for example, is placed above theear for radiating electromagnetic (EM) energy through the skin and intoreceiving antenna 1004 implanted just under the skin, as illustrated inFIG. 10B. The simulation incorporates model equations into the highfrequency structural simulator to investigate the process of coupling EMenergy from bowtie antenna assembly 1000 outside the skin to receivingantenna 1006 implanted at the skull outer surface. The simulationresults demonstrated below are presented in the coronal plane thatintersects center of the transmitting antenna 1000 and receiving antenna1006.

As an initial matter, FIG. 11A shows the dimensions of the examplebowtie antenna assembly 1100 as used in the stimulation investigations.Bowtie antenna assembly 1100 may generally include signal metal layerand a feed port, as discussed above in association with FIG. 3B. Thebowtie antenna assembly 1100 includes two leaves, namely 1102L and1102R, each of which has a width W at the widest point of 25.4 mm. Thecombined length L of leaves 1100L and 1100R is 85 mm. The two leaves1102L and 1102R form an angle of D. Here, the bowtie antenna assembly1100 functions like a dipole transmitting antenna and the geometricalparameters of L, W, and D can determine the resonance behavior of bowtieantenna assembly 1100. During operation, the two leaves of bowtieantenna assembly 1100 may respectively function as a signal arm and aground arm at any given time. The long axis of the two-leaf structuredetermines the direction of the linear polarization. As illustrated,leaf 1102L includes rounded fillets 1102A and 1102B while leaf 1100Rincludes rounded fillets 1102C and 1102D. Rounded fillets 1102A to 1102Bare rounded with a 10 mm radius. Further, the two leaves 1102L and 1102Rconverge at 1102M where a 50Ω feed port can be located. In other words,the two leaves 1102L and 1102R adjoin at vertex 1100M. The feed port mayconnect bowtie antenna assembly 700 to, for example, a signal generator,such as RF pulse generator 106. By way of example, BNC (BonetNeill-Concelman) or SMC (SubMiniature version A) type connectors can beused to connect 50Ω feed port to an MFS device through a co-axial cableThe bowtie antenna length can vary between 50 to 120 cm, or greater.

FIG. 11B shows the simulated S11 parameter (input port voltagereflection coefficient) for the bowtie antenna assembly 1100. There isan approximate 8.8 dB match at 915 MHz. Accordingly, the antennaassembly 1000 is configured to have a reflection ratio of at least 8 dBat the operating frequency. An 8.8 dB or more suppression means lessthan 14% of the transmitted energy may get reflected. In other words,more than 86% of the transmitted energy passes through. The antennaassembly 1000 may be configured such that the reflection ratio of atleast 8 dB at the operating frequency of the antenna assembly ismaintained when the antenna assembly is positioned between zero to 2centimeters, or more particularly, between zero to 1 centimeter, awayfrom the patient's skin. That is, the antenna assembly 1000 may beconfigured such that the S11 parameter notch at the operating frequencyis wide enough that the S11Ω parameter remains below −8 dB as theantenna assembly 1000 is positioned between 0 to 2 centimeters, or moreparticularly, between zero to 1 centimeter, away from the patient'sskin.

FIG. 12A shows an example of a radiation pattern from the bowtie antennaassembly of FIGS. 10A-10C in a sagittal view. Here, an example of thesimulated specific absorption rate (SAR) resulting from the transcranialstimulation is presented at the bowtie antenna assembly 1000. The effectof a variety of parameters on the SAR patterns can be simulated. Thereseparameters may generally include average input power, maximum peakpower, and duty cycle. Average input power may be in the range of 20 mWto 80 mW. Peak power may be in the range of 2 to 4 W. Duty cycle may bein the range of 0.5% to 4%. Various SAR patterns demonstrate that theradiated EM field can penetrate through the skull for transcranialdelivery of EM energy. Further, the penetration of the radiated EM fieldcan be improved as the input power at the bowtie antenna assembly 1000is ramped up from 20 mW to 80 mW.

FIG. 12B shows the example radiation pattern of FIG. 12A in a zoomedview and superposed with the profile of the receiving antenna 1004, whenthe average input power at the bowtie antenna assembly 1000 is 80 mW. Inthis example, there is a SAR of 8 W/kg around the region occupied by thecross-sectional profile of the receiving antenna 1004. The area in whichthe SAR numbers are at the maximum allowable number may also be known asthe hot spot. Here, the hot spot is about 4 mm under the skull, whichgenerally coincides with the profile of the receiving antenna 1004.

FIG. 13 shows the example of the bowtie antenna assembly 1000 inposition for radiative treatment in an axial view. As illustrated, thebowtie antenna assembly 1000 also includes a coaxial feed 1002 andconfigured to take on, for example, a 50Ω load. Here, the illustrationin FIG. 13 also shows the mesh grid of a finite element model used to insimulation experiments to investigate performance of bowtie antennaassembly 1000, as discussed above in association with FIGS. 10A-10C.

FIG. 14A shows an example of a radiation pattern from the bowtie antennaassembly of FIGS. 10A-10D in the axial view that captures the plane ofthe receiving antenna 1004. Here, the simulated specific absorption rate(SAR) resulting from the transcranial stimulation can be generated forvarious average input powers at the bowtie antenna assembly 1000, asdiscussed above in association with FIG. 12A. For example, the averageinput power can vary from 20 mW to 80 mW. The peak input power may be 2W or greater, for example, from 2 W to 4 W. The duty cycle may changefrom 0.5% to 4%. Simulation results do demonstrate that the radiated EMfield can penetrate through the skull for transcranial delivery of EMenergy. The penetration of the radiated EM field can improve as theinput power at the bowtie antenna assembly 1000 is ramped.

FIG. 14B shows an example of a radiation pattern from FIG. 14A in azoomed view and superposed with the plane of the receiving antenna 1004,when the average input power at the bowtie antenna assembly 1000 is 80mW. As illustrated, at 8 W/kg is generated around the region towards themiddle of the plane of the receiving antenna 1004. The area in which theSAR numbers are at the maximum allowable number may also be known as thehot spot. Here, the hot spot is about 1.1 cm under the skull, whichgenerally coincides with the plane of the receiving antenna 1004. Insome instances, the bowtie configuration may be more likely to generatea higher concentration of electric field in the near field than a widedipole configuration with comparable surface area. Here, higherconcentration of electric filed may correspond to a larger amplitude inmeasured electric field strength both in terms of peak amplitude andmean amplitude. In this respect, the bowtie configuration may be moreadvantageous for emitting linearly polarized electromagnetic energy to areceiving dipole antenna.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made. Accordingly, otherimplementations are within the scope of the following claims.

What is claimed is:
 1. A system, comprising: a signal generatorconfigured to generate an input signal containing electrical energy andstimulation pulse parameters; and an antenna assembly, comprising: ametal layer configured to emit linearly polarized electric field to areceiving antenna implanted underneath a subject's skin such that thereceiving antenna implanted underneath the subject's skin wirelesslyreceives the linearly polarized electric field emitted from the antennaassembly; and a feed port configured to connect the antenna assembly tothe signal generator such that the antenna assembly receives an inputsignal from the signal generator and then drives the metal layer usingthe input signal to emit the linearly polarized electric field to thereceiving antenna, wherein the metal layer is a two-leaf structure thatincludes two leaves each having three vertices, wherein the two leavesadjoin each other at one vertex, and the metal layer is operable tocreate an electric field density higher than that created by a differentmetal layer configured differently than the two-leaf structure but witha surface area identical to the two-leaf structure, and wherein theantenna assembly operates with a reflection ratio of at least 6 dB, thereflection ratio corresponding to a ratio of a transmission power of theantenna assembly when driven by the input signal and a reflection powerseen by the antenna assembly resulting from emission of the linearlypolarized electric field to the receiving antenna implanted underneath asubject's skin.
 2. The system of claim 1, wherein the remaining verticesof each leaf are rounded as fillets.
 3. The system of claim 2, whereinthe feed port is located at the vertex where the two leaves adjoin eachother.
 4. The system of claim 1, wherein a long axis of the two-leafstructure aligns with a direction of the linear polarized electricfield.
 5. The system of claim 1, wherein the metal layer is less than200 μm in thickness.
 6. The system of claim 1, wherein the metal layerincludes four rounded fillets.
 7. The system of claim 1, wherein thefeed port is located on a midline of a surface of the rectangularstructure.
 8. The system of claim 1, wherein the antenna assembly isconfigured such that the antenna assembly can be bent up to 50 degreeswhile maintaining the reflection ratio of at least 6 dB.
 9. The systemof claim 1, wherein the antenna assembly is configured to emittranscranially the linearly polarized electric field when the antennaassembly is worn as part of an ear piece.
 10. The system of claim 1,wherein the metal layer is less than 200 μm in thickness.
 11. Theantenna assembly of claim 1, wherein the antenna assembly is configuredsuch that the reflection ratio of at least 6 dB is maintained regardlessof a separation between the metal layer and a subject's skin.
 12. Thesystem of claim 1, wherein the antenna assembly is configured such thatthe reflection ratio of at least 6 dB is maintained with an air gapbetween the metal layer and the subject's skin.
 13. The system of claim1, wherein the antenna assembly is configured to operate with a qualityfactor (Q) no more than
 9. 14. The system of claim 1, wherein theantenna assembly is configured to operate at a frequency between 300 MHzand 8 GHz.
 15. The system of claim 1, further comprising: a passiveneural stimulator device comprising: the receiving antenna configured toreceive the input signal emitted from the antenna assembly; andcircuitry coupled to the receiving antenna and configured to: extractelectric energy contained in the input signal; and create, using theextracted electric energy, stimulation pulses suitable for stimulatingneural tissue.
 16. The system of claim 15, wherein the passive neuralstimulator device further comprises one or more electrodes for applyingthe stimulation pulses for stimulation neural tissue.
 17. An antennaassembly, comprising: a metal layer configured to emit linearlypolarized electric field to a receiving antenna implanted underneath asubject's skin such that the receiving antenna implanted underneath thesubject's skin wirelessly receives the linearly polarized electric fieldemitted from the antenna assembly; and a feed port configured to connectthe antenna assembly to a signal generator such that the antennaassembly receives an input signal from the signal generator and thendrives the metal layer using the input signal to emit the linearlypolarized electric field to the receiving antenna, wherein the metallayer is a two-leaf structure that includes two leaves each having threevertices, wherein the two leaves adjoin each other at one vertex, andthe metal layer is operable to create an electric field density higherthan that created by a different metal layer configured differently thanthe two-leaf structure but with a surface area identical to the two-leafstructure; and wherein the antenna assembly operates with a reflectionratio of at least 6 dB, the reflection ratio corresponding to a ratio ofa transmission power of the antenna assembly when driven by the inputsignal and a reflection power seen by the antenna assembly resultingfrom emission of the linearly polarized electric field to the receivingantenna implanted underneath a subject's skin.
 18. The system of claim17, further comprising: a passive neural stimulator device comprising:the receiving antenna configured to receive the input signal emittedfrom the antenna assembly; and circuitry coupled to the receivingantenna and configured to: extract electric energy contained in theinput signal; and create, using the extracted electric energy,stimulation pulses suitable for stimulating neural tissue.
 19. Thesystem of claim 17, wherein the antenna assembly is configured such thatthe reflection ratio of at least 6 dB is maintained while the antennaassembly is not touching the subject's skin.